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- Open a unique background text field on Task object.
- Create a workflow / process that is defined on Task object, select "only when record is created"
- Set LEFT([Task].WhatId ,3)="500" as your criteria. 500 Here is the object ID of Case object.
- Update the background field with the value of address field as an immediate action. Any duplicate address value should throw out an error and prevent the creation of task record.